1. Field of the Invention
The present invention relates to a method and device for controlling the turn-OFF of a solid state switch, and especially a thyristor. GTO or SGTO.
2. Background of the Related Art
Thyristors are turned ON by a gate signal. Once the gate signal is removed, the thyristor remains in the ON-state until the current flowing through the thyristor falls below a certain threshold value. Thus, Gate Turn-Off Thyristors (GTOs) can be turned ON by a gate signal of a positive current pulse between the gate and cathode terminals, and turned OFF by a gate signal of negative polarity between the gate and cathode terminals. However. GTOs suffer from long switch OFF times. After the turn-OFF current terminates, there is a long tail time where residual current continues to flow until all remaining charge from the device dissipates.
A number of devices have been developed to aid in the turn-OFF of thyristors, such as discussed in U.S. Pat. No. 6,191,640 to Coenraads et al., U.S. Pat. No. 6,597,555 to Gruening et al., U.S. Pat. No. 4,868,731 to Hobi, and U.S. Pat. No. 5,514,921 to Steigerwald. However, these patents do not improve the turn-OFF characteristics of the solid state switches. In addition the assignee, Silicon Power, turns OFF its Super Gate Turn-Off Thyristor (SGTO) by applying a −9 volt pulse.
One of the factors affecting the turn-OFF characteristics of the solid state switches is the gate current DIg/DT value which is limited by the gate drive circuit inductance, the gate inductance of the device and by the reverse blocking voltage capability of the gate-cathode junction (in the present case by −9V). So a turn-OFF pulse that exceeds the reverse blocking voltage capability of the gate-cathode junction (in our case by −9V) continuously will put the junction of the thyristor into avalanche breakdown and the gate drive will supply high current through junction. This will affect the solid state switches temperature and switches can be destroyed. To avoid that, designers will apply a −9 volt pulse to turn OFF a thyristor. Consequently, there is still a need to increase the turn-off current value and reduce the OFF times for GTOs.